1. Field of the Invention
The present invention relates to a method of fabricating a thin film transistor and, more particularly, to a method of fabricating a thin film transistor in which, in order to control the concentration of metal catalysts remaining on a polycrystalline silicon layer when an amorphous silicon layer formed on an insulating substrate is crystallized into the polycrystalline silicon layer using a super grain silicon (SGS) crystallization method, the substrate is annealed so that a very small amount of metal catalyst is adsorbed or diffused into a capping layer, and then a crystallization process is carried out.
2. Description of Related Art
Generally, in a thin film transistor for use in display devices, an amorphous silicon is deposited on a transparent substrate of glass, quartz or the like, is dehydrogenated, and then is crystallized to form a semiconductor layer.
At this time, the semiconductor layer, which constitutes source, drain and channel regions of the thin film transistor, is formed by depositing the amorphous silicon layer on the transparent substrate such as glass using a chemical vapor deposition (CVD) method. However, the silicon layer directly deposited on the substrate by, for example, the CVD method has low electron mobility because it contains hydrogen of about 12%. In addition, when the amorphous silicon layer having such low electron mobility is annealed and crystallized into a silicon layer of a crystalline structure having high electron mobility, the silicon layer is damaged by the cracking of the contained hydrogen. A dehydrogenation process is carried out to prevent the cracking phenomenon of the hydrogen generated upon the crystallization. The dehydrogenation process is generally performed by annealing at a temperature of about 400° C. or more for tens of minutes to a few hours in a furnace. A crystallization process is then carried out to crystallize the dehydrogenated amorphous silicon layer.
A method of crystallizing the amorphous silicon into polysilicon includes a solid phase crystallization method, an excimer laser crystallization method, a metal induced crystallization method, a metal induced lateral crystallization method, and the like. The solid phase crystallization method is a method of annealing an amorphous silicon layer over a few hours to tens of hours at a temperature about 700° C. or less that is a transition temperature of glass, which is a material forming a substrate of a display device that uses thin film transistors. The excimer laser crystallization method is a method in which a silicon layer is irradiated by an excimer laser, and heated locally at a high temperature for a very short time period to crystallize. The metal induced crystallization method is a method that uses a phenomenon that amorphous silicon is crystallized to polysilicon through a phase change by a metal, such as nickel, palladium, gold, aluminum or the like, which comes in contact with or is injected into an amorphous silicon layer. The metal induced lateral crystallization method is a method of crystallizing a silicon layer using a method in which silicide created by the reaction of metal and silicon is propagated continuously and laterally to induce the sequential crystallization of silicon.
However, the solid phase crystallization method has a disadvantage that a substrate is easily deformed due to too long processing time and long-time annealing at a high temperature. The excimer laser crystallization method has a disadvantage that it needs an expensive laser apparatus, as well as an interface property between a semiconductor layer and a gate insulating layer is bad due to extrusions created on a polycrystallized surface. In the case of crystallization that uses the metal induced crystallization method or the metal induced lateral crystallization method, there is a disadvantage that metal catalysts remain on a crystallized polycrystalline silicon layer to increase a leakage current in a semiconductor layer of a thin film transistor.